1. Technical Field
The systems and methods disclosed herein relate to the field of performing error correction and, more specifically, to systems and methods of performing concatenated error correction on data stored in non-volatile memory.
2. Description of the Related Art
Non-volatile memory, such as flash EEPROM memory and/or NAND flash memory, is used in a variety of electronic devices. For example, non-volatile memory may be used in a cellular phone, a personal digital assistant, a radio, a personal computer, a notebook, a workstation, a camera, an audio player, a global positioning device, a server, and other devices. Non-volatile memory may include a number of cells at different levels, where the cells may be programmed to store information as a series of binary digits (0 and 1) or “bits.”
As with most electronic devices, non-volatile memory devices are susceptible to defects and errors. Errors may result from factors including the shifting of threshold levels of the memory states as a result of normal operations of the memory device including write/program, erase, and/or read operations. For example, during a program operation electrons may be added to charge the cell to reflect a particular state.
During programming of a memory cell, it is more difficult to transfer multiple discrete units of charge (electrons) to the memory cell than it is to simply fully charge or discharge the memory cell. Moreover, once the programming operation begins and electrons are being added, it is difficult to remove charge without performing an erase function. This approach may cause a cell to be programmed incorrectly and may cause “overshoot” errors—i.e., the cells that are adjacent to the cell being programmed may receive extra electrons that should have been added to the cell being charged. Since, electrons may not be removed easily, the errors caused due to overshoot may be referred to as an “asymmetric error” or a “one-directional error,” and error correction code (ECC) techniques may be used to correct these errors.
In conventional systems, ECC techniques involve generating redundant bits (parity bits) that are added to the end of data strings during program operations. These redundant bits may then be utilized to detect and correct errors as data is read out from the non-volatile memory. However, conventional techniques involve the use of asymmetric error code, where code words consisting of asymmetric error correction code may be included in the end of data strings. The asymmetric error correction code used in conventional systems usually only corrects one directional error types. In real systems, errors are not strictly one directional, rendering such codes of little practical value.